Alkyl polyglycerol emulsion compositions for fuel cell reformer start-up

ABSTRACT

The present invention relates to emulsion compositions for starting a reformer of a fuel cell system. In particular, the invention includes emulsion compositions comprising hydrocarbon fuel, water and alkyl polyglycerol surfactants for starting a reformer of a fuel cell system.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/352,021 filed Jan. 25, 2002

BACKGROUND OF INVENTION

The present invention relates to compositions for use at start-up areformer of a fuel cell system. In particular, this invention includesemulsion compositions comprising hydrocarbon fuel, water and surfactantfor use at start-up of a reformer of a fuel cell system.

Fuel cell systems employing a partial oxidation, steam reformer orautothermal reformer or combinations thereof to generate hydrogen from ahydrocarbon need to have water present at all times to serve as areactant for reforming, water-gas shift, and fuel cell stackhumidification. Since water is one product of a fuel cell stack, duringnormal warmed-up operation, water generated from the fuel cell stack maybe recycled to the reformer. For start-up of the reformer it ispreferable that liquid water be well mixed with the hydrocarbon fuel andfed to the reformer as an emulsion. The current invention providesemulsion compositions suitable for use at start-up of a reformer of afuel cell system.

SUMMARY OF THE INVENTION

One embodiment of the invention provides emulsion compositions suitablefor use at start-up of a reformer of a fuel cell system comprisinghydrocarbon, water and surfactant.

In a preferred embodiment, the emulsion composition is a bicontinuousemulsion comprising a coexisting mixture of at least 60 vol % of ahydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion.

In another embodiment of the invention is provided a method to prepare abicontinuous emulsion comprising a coexisting mixture of at least 60 vol% of a hydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion comprising mixing hydrocarbon, waterand surfactant at low shear.

In yet another embodiment is a bicontinuous emulsion compositioncomprising a coexisting mixture of at least 60 vol % of ahydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a typical prior art conventionalfuel cell system.

FIG. 2 shows a schematic diagram of an improved fuel cell system whereina start-up system is operably connected to a reformer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The emulsion compositions of the present invention can be used forstart-up of a reformer of a fuel cell system. In a preferred embodimentthe emulsion compositions can be used for start-up of a reformer of animproved fuel cell system described hereinafter. The improved fuel cellsystem comprises a convention fuel cell system to which a start-upsystem is operably connected. A conventional fuel cell system and theimproved fuel cell system are described below.

A conventional fuel cell system comprises a source of fuel, a source ofwater, a source of air, a reformer, a water gas shift reactor, reactorsfor converting CO to CO₂ and a fuel cell stack. A plurality of fuelcells operably connected to each other is referred to as a fuel cellstack. FIG. 1 shows a schematic of one embodiment of a prior arthydrogen generator based on a hydrocarbon liquid fuel and using partialoxidation/steam reforming to convert the fuel into a syngas mixture.This system design is similar to that being developed by A. D. Little,except for the allowance of feeding water to the reformer to practiceautothermal reforming (Ref.: J. Bentley, B. M. Barnett and S. Hynke,1992 Fuel Cell Seminar-Ext. Abs., 456, 1992). The process in FIG. 1 iscomprised as follows: Fuel is stored in a fuel tank (1). Fuel is fed asneeded through a preheater (2) prior to is entering the reformer (3).Air is fed to the reformer (3) after it is heated in the preheater (5).Water is stored in a reservoir tank (6). A heat exchanger (7) isintegral with a portion of tank (6) and can be used to melt portions ofthe water if it should freeze at low operation temperatures. Some waterfrom tank (6) is fed via stream (9) to preheater (8) prior to enteringthe reformer (3). The reformed syngas product is combined withadditional water from tank (6) via stream (10). This humidified syngasmixture is then fed to reactors (11) which perform water gas shift(reaction of CO and water to produce H₂) and CO cleanup. The H₂rich-fuel stream then enters the fuel cell (12) where it reactselectronically with air (not shown) to produce electricity, waste heatand an exhaust stream containing vaporized water. A hydrogen-oxygen fuelcell as used herein includes fuel cells in which the hydrogen-rich fuelis hydrogen or hydrogen containing gases and the oxygen may be obtainedfrom air. This stream is passed through a condenser (13) to recover aportion of the water vapor, which is recycled to the water reservoir (6)via stream (14). The partially dried exhaust stream (15) is released tothe atmosphere. Components 3 (reformer) and 11 (water gas shift reactor)comprise a generalized fuel processor.

FIG. 2 shows a schematic of one configuration for the fuel cell start-upsystem for connection to the conventional fuel cell system. The systemin FIG. 2 is comprised as follows: fuel is stored in a fuel container(1), water in a water container (2), antifreeze in an antifreezecontainer (3), surfactant in a surfactant container (4), and emulsion ismade in an emulsion container (5). The fuel and surfactant containers(1) and (4) are connected to the emulsion container (5) via separatetransfer lines (6) and (7) respectively. The water container (2) isconnected to the emulsion container (5) via a transfer line (8) todispense water or water-alcohol mixture to the emulsion container. Thewater container is further connected to an antifreeze container (3) viaa transfer line (9). The emulsion container is fitted with a mixer. Anoutlet line (10) from the emulsion is container (5) is connected to thefuel cell reformer of a conventional system such as a reformer (3) shownin FIG. 1; (reformer (3) of FIG. 1 is equivalent to reformer (11) shownin FIG. 2). The fuel, water and surfactant containers are allindividually connected to a start-up microprocessor (12) whose signalinitiates the dispensing of the fuel, water and surfactant into theemulsion container. The water container is connected to a temperaturesensor (13), which senses the temperature of the water in the watercontainer. The temperature sensor is connected to a battery (not shown)and the antifreeze container. The temperature sensor triggers theheating of the water container or dispensing of the antifreeze asdesired. The configuration for the fuel cell start-up described above isone non-limiting.

In an alternate embodiment of the start-up system the water container isthe water storage chamber of the conventional fuel cell system. Inanother embodiment of the start-up system the emulsion container iseliminated. Fuel, water and surfactant are dispensed directly into thetransfer line (10) shown in FIG. 2. In this embodiment the transfer line(10) is fitted with in-line mixers. A typical in-line mixer is comprisedof a tubular container fitted with in-line mixing devices known in theart. One non-limiting example of an in-line mixing device is a series offins attached perpendicular to the fluid flow. Another example is aseries of restricted orifices through which fluid is propagated. In-linemixers are known to those skilled in the art of mixing fluids. Theplacement of the number and angle of the fins to the circumference ofthe tube is known to those skilled in the art of in-line mixer design. Asonicator can also be used as an in-line mixing device. The sonicatordevice for in-line mixing comprises a single sonicator horn or aplurality of sonicator horns placed along the transfer line (10).

A mixture comprising fuel and surfactant can be simultaneously injectedwith water into the front portion of the in-line mixer. Alternately, amixture comprising water and surfactant can be simultaneously injectedwith fuel into the front portion of the in-line mixer. The fuel, waterand surfactant are mixed as they flow through the in-line mixer to forman emulsion. The end portion of the in-line mixer delivers the emulsionto the reformer through an injection nozzle.

One function of the improved fuel cell system is that at start-up, thefuel and water are delivered as an emulsion to the reformer. Oneadvantage to using an emulsion at start-up is that a well-mixedwater/fuel injection is achieved. This can improve the efficiency ofstart-up of the reformer. Another advantage of using an emulsion is thatthe fuel-water mixture can be sprayed into the reformer as opposed tointroducing vapors of the individual components into the reformer.Delivery of the fuel and water as an emulsion spray has reformerperformance advantages over delivery of the fuel and water in avaporized state. Further spraying the emulsion has mechanical advantagesover vaporizing the components and delivering the vapors to thereformer. Among the desirable features of emulsions suitable for use inthe improved fuel cell start-up system described herein are: (a) theability to form emulsions are low shear; (b) the ability of thesurfactants to decompose at temperatures below 700° C.; (c) theviscosity of the emulsions being such that they are easily pumpable; and(d) the emulsion viscosity decreases with decreasing temperature. Theemulsions of the instant invention possess these and other desirableattributes.

The fluid dispensed from the emulsion container or the in-line mixerinto the reformer is the emulsion composition of the instant inventionsuitable for start-up of a reformer of a fuel cell system. Once thereformer is started with the emulsion composition it can continue to beused for a time period until a switch is made to a hydrocarbon and steamcomposition. Typically a start-up time period can range from 0.5 minutesto 30 minutes depending upon the device the fuel cell system is thepower source of. The emulsion composition of the instant inventioncomprises hydrocarbon, water and surfactant. In a preferred embodimentthe emulsion further comprises low molecular weight alcohols. Anotherpreferred embodiment of the emulsion composition is a bicontinuousemulsion comprising a coexisting mixture of at least 60 vol % of ahydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion.

A hydrocarbon-in-water emulsion is one where hydrocarbon droplets aredispersed in water. A water-in-hydrocarbon emulsion is one where waterdroplets are dispersed in hydrocarbon. Both types of emulsions requireappropriate surfactants to form stable emulsions of the desired dropletsize distribution. If the average droplet sizes of the dispersed phaseare less than about 1 micron in size, the emulsions are generally termedmicro-emulsions. If the average droplet sizes of the dispersed phasedroplets are greater than about 1 micron in size, the emulsions aregenerally termed macro-emulsions. A hydrocarbon-in-water macro or microemulsion has water as the continuous phase. A water-in-hydrocarbon macroor micro emulsion has hydrocarbon as the continuous phase. Abicontinuous emulsion is an emulsion composition whereinhydrocarbon-in-water and water-in-hydrocarbon emulsions coexist as amixture. By “coexist as a mixture” is meant that the microstructure ofthe emulsion fluid is such that regions of hydrocarbon-in-waterintermingle with regions of water-in-hydrocarbon. A bicontinuousemulsion exhibits regions of water continuity and regions of hydrocarboncontinuity. A bicontinuous emulsion is by character amicro-heterogeneous biphasic fluid.

The hydrocarbon component of the emulsion composition of the instantinvention is any hydrocarbon boiling in the range of 30° F. (−1.1° C.)to 500° F. (260° C.), preferably 50° F. (10° C.) to 380° F. (193° C.)with a sulfur content less than about 120 ppm and more preferably with asulfur content less than 20 ppm and most preferably with a no sulfur.Hydrocarbons suitable for the emulsion can be obtained from crude oilrefining processes known to the skilled artisan. Low sulfur gasoline,naphtha, diesel fuel, jet fuel, kerosene are non-limiting examples ofhydrocarbons that can be utilized to prepare the emulsion of the instantinvention. A Fisher-Tropsch derived paraffin fuel boiling in the rangebetween 30° F. (−1.1° C.) and 700° F. (371° C.) and, more preferably, anaphtha comprising C5-C10 hydrocarbons can also be used.

The water component of the emulsion composition of the instant inventionis water that is substantially free of salts of halides, sulfates andcarbonates of Group I and Group II elements of the long form of ThePeriodic Table of Elements. Distilled and deionoized water is suitable.Water generated from the operation of the fuel cell system is preferred.Water-alcohol mixtures can also be used. Low molecular weight alcoholsselected from the group consisting of methanol, ethanol, normal andiso-propanol, normal, iso- and secondary-butanol, ethylene glycol,propylene glycol, butylene glycol and mixtures thereof are preferred.The ratio of water:alcohol can vary from about 99.1:0.1 to about 20:80,preferably 90:10 to 70:30.

An essential component of the emulsion composition of the instantinvention is at least one surfactant selected from the group consistingof alkyl polyglycerol mono ester and alkyl polyglycerol diester andmixtures thereof and represented by the respective formula

R—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂OH

and

R—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂—O—CO—R

where R is an alkyl group of 2 to 26 carbon atoms and n is an integerfrom about 0 to 15 and mixtures thereof.

The term “alkyl” in the alkyl polyglycerol mono ester and alkylpolyglycerol diester surfactants is meant to represent saturated alkylhydrocarbons, unsaturated alkyl hydrocarbons or mixtures thereof. Thealkyl group can be linear or branched. The preferred surfactants arethermally labile and decompose in the temperature range of 250° C. to700° C. Preferably at about 700° C. substantially all of the surfactantis decomposed. The total concentration of surfactants in the emulsioncomposition is in the range of 0.01 to 5 wt %. The preferred surfactantconcentration is in the range of 0.05 to 1 wt %.

The ratio of hydrocarbon:water in the emulsion can vary from 40:60 to60:40 based on the weight of the hydrocarbon and water. In terms of theratio of water molecule:carbon atom in the emulsion, the ratio can be0.25 to 3.0. A ratio of water molecule:carbon atom of 0.9 to 1.5 ispreferred.

It is preferred to store the surfactant as a concentrate in the start-upsystem of the fuel cell reformer. The surfactant concentrate cancomprise the said surfactant or mixtures of said surfactants andhydrocarbon. Alternately, the surfactant concentrate can comprise thesaid surfactant or mixtures of said surfactants and water. The amount ofsurfactant can vary in the range of about 80% surfactant to about 30 wt%, based on the weight of the hydrocarbon or water. Optionally, thesurfactant concentrate can comprise the said surfactant or mixtures ofsaid surfactants and a water-alcohol solvent. The amount of surfactantscan vary in the range of about 80 wt % to about 30 wt %, based on theweight of the water-alcohol solvent. The ratio of water:alcohol in thesolvent can vary from about 99:1 to about 1:99. The hydrocarbon, waterand alcohol used for storage of the surfactant concentrate arepreferably those that comprise the emulsion and described in thepreceding paragraphs.

The surfactants of the instant invention when mixed with hydrocarbon andwater at low shear form a bicontinuous emulsion. Low shear mixing can bemixing in the shear rate range of 1 to 50 sec⁻¹, or expressed in termsof mixing energy, in the mixing energy range of 0.15×10⁻⁵ to 0.15×10⁻³kW/liter of fluid. Mixing energy can be calculated by one skilled in theart of mixing fluids. The power of the mixing source, the volume offluid to be mixed and the time of mixing are some of the parameters usedin the calculation of mixing energy. In-line mixers, low shear staticmixers, low energy sonicators are some non-limiting examples for meansto provide low shear mixing.

A method to prepare the emulsion of the instant invention comprises thesteps of adding surfactant to the hydrocarbon phase, adding the saidsurfactant solution to water and mixing at a shear rate in the range of1 to 50 sec⁻¹ (0.15×10⁻⁵ to 0.15×10⁻³ kW/liter of fluid) for 1 second to15 minutes to form the bicontinuous emulsion mixture. Optionally, thesurfactant may be added to water and the solution added to hydrocarbonfollowed by mixing. Another method to prepare the emulsion comprisesadding the water-soluble surfactant to the water phase,hydrocarbon-soluble surfactant to the hydrocarbon phase and then mixingthe aqueous surfactant solution with the hydrocarbon surfactantsolution. Yet another method comprises adding the surfactants to thehydrocarbon-water mixture followed by mixing.

In a preferred embodiment, the reformer of the fuel cell system isstarted with a bicontinuous emulsion comprising a coexisting mixture ofat least 60 vol % of a hydrocarbon-in-water macro emulsion and from 1 to40 vol % of a water-in-hydrocarbon macro emulsion. When a mixture ofhydrocarbon, water or water-methanol mixtures and surfactants of theinstant invention are subject to low shear mixing a bicontinuousemulsion comprising a mixture of at least 60 vol % of ahydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion is formed.

When alkyl polyglycerol mono esters and alkyl polyglycerol diesters ofthe structures shown in structures 1 and 2 below are added to naphthaand distilled water and subject to low shear mixing bicontinuousemulsions are formed. Further, substitution of water with water/methanolmixture in the ratio of 80/20 to 60/40 does not alter the emulsifyingperformance of the surfactants or the nature of bicontinuous emulsionthat is formed. A single surfactant selected from the group shown instructure 1 or 2 may be used. It is preferred to use a mixture ofwater-soluble and hydrocarbon soluble surfactants of the type shown instructures 1 and 2.

Structure 1: Alkyl Polyglycerol Mono Ester

R—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂OH

where R is an alkyl group of 2 to 26 carbon atoms and n is an integerfrom about 0 to 15 and mixtures thereof.

Structure 2: Alkyl Polyglycerol Diester

R—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂—O—CO—R

where R is an alkyl group of 2 to 26 carbon atoms and n is an integerfrom about 0 to 15 and mixtures thereof.

When a mixture of surfactants of the types shown in structures 1 and 2is used the ratio of structure 1 surfactant:structure 2 surfactant canvary in the range of 95:5 to 5:95 by weight.

In the operation of the fuel cell it is expected that the emulsioncomposition will be utilized at start-up of the reformer and extendingfor a time period when a switch to hydrocarbon and steam is made. Oneembodiment of the invention is the feeding to the reformer of a fuelcell system, first a composition comprising the emulsion composition ofthe instant invention, followed by a hydrocarbon/steam composition. Thebicontinuous emulsion composition allows a smooth transition to thehydrocarbon/steam composition.

The following non-limiting examples illustrate the invention.

EXAMPLE 1

The effectiveness of the surfactants to form emulsions is expressedquantitatively by the reduction in interfacial tension between thehydrocarbon and water phases. Naphtha, a hydrocarbon mixture distillingin the boiling range of 50° F. to −400° F. or 10° C. to 204° C. was usedas the hydrocarbon and double distilled deionized water as the aqueousphase. Interfacial tensions were determined by the pendant drop methodknown in the art. Greater than 80% reduction in interfacial tension wasobserved indicative of the propensity for emulsification of the waterand hydrocarbon phases by these surfactants. Table 1 providescomparative interfacial tension data.

TABLE 1 Solution Interfacial tension (dynes/cm) Naphtha / Water 53.02Naphtha / Water + 7.0 1 wt % alkyl polyglycerol mono ester (Structure 1,R = C17; n = 1)

Thermogravimetry experiments were conducted on representativesurfactants shown in structure 1 (R═C17; n=1). It was observed that thesurfactant decomposed in the temperature range of 250° C. to 700° C.Substantially all of the surfactants had decomposed at a temperature ofabout 400° C.

EXAMPLE 2

1 g of alkyl polyglycerol surfactant (Structure 1, R═C17; n=1) (sold byAbietic Corporation as Caprol 3GO) was added to a mixture of 50 gnaphtha (dyed orange) and 50 g water (dyed blue) and mixed using aFisher Hemetology/Chemistry Mixer Model 346. Mixing was conducted for 5minutes at 25° C.

Conductivity measurements are ideally suited to determine the phasecontinuity of an emulsion. A water continuous emulsion will haveconductivity typical of the water phase. A hydrocarbon continuousemulsion will have negligible conductivity. A bicontinuous emulsion willhave a conductivity intermediate between that of water and hydrocarbon.

By using dyes to color the hydrocarbon and water, optical microscopyenables determination of the type of emulsions by direct observation.The third technique to characterize emulsions is by determination ofviscosity versus shear rate profiles for the emulsion as a function oftemperature.

Using a Leitz optical microscope the emulsion of Example 2 wascharacterized as a mixture of hydrocarbon-in-water macro type incoexistence with a water-in-hydrocarbon macro type. Thehydrocarbon-in-water type emulsion was the larger volume fraction of themixture.

A measured volume of the emulsion of Example 2 was poured into agraduated vessel and allowed to stand for about 72 hours. Theco-existing bicontinuous emulsion mixture separated, after 72 hours ofstanding, into the constituent emulsion types. The hydrocarboncontinuous type was the upper phase and the water continuous type thelower phase. The graduated vessel allowed quantitative determination ofthe volume fraction of each type of emulsion.

The conductivity of water was recorded as 47 micro mho; naphtha as 0.1micro mho and the emulsion of Example 2 was 35 micro mho confirming thebicontinuous emulsion characteristics of the fluid.

Viscosity as a function of shear rate was determined for the emulsion ofExample 2 at 25° C. and 50° C. A decrease in viscosity with decreasingtemperature was observed. An emulsion exhibiting decreasing viscositywith decreasing temperatures is unique and advantageous for lowtemperature operability of the reformer.

Further, the emulsion of Example 2 was stable for at least 12 hours at25° C. in the absence of shear or mixing. In comparison, in a controlexperiment wherein the stabilizing surfactants were omitted and only thehydrocarbon and water were mixed, the resulting emulsion phase separatedwithin 5 seconds upon ceasing of mixing. Yet another unexpected featureof the emulsions of the instant invention is that when the emulsionswere cooled to −54° C. they solidified and when thawed or heated to +50°C. the emulsions liquefied and retained their stability and bicontinuousnature. This is in contrast to single-phase continuity emulsions thatphase separate upon cooling and thawing.

Using stable bicontinuous emulsions comprised of hydrocarbon, water andsuitable surfactants has reformer performance advantages andenhancements compared to using unstable emulsions of hydrocarbon andwater in the absence of stabilizing surfactants as disclosed in U.S.Pat. No. 5,827,496. The stability, bicontinuous characteristic and theobserved decrease in viscosity with decreasing temperature are at leastthree distinguishing features of the emulsion composition of the instantinvention that can result in unexpected enhancement in reformerperformance compared to conventional unstable emulsions withsingle-phase continuity and increasing viscosity with decreasingtemperature.

What is claimed is:
 1. In a fuel cell system comprising a reformer toproduce a hydrogen containing gas for use in a fuel cell stack, theimprovement comprising: feeding to the reformer, at start-up, anemulsion composition comprising at least 40 wt % of hydrocarbon, from 30to 60 wt % of water, and from 0.01 to 5 wt % of at least one surfactantselected from the group consisting of alkyl polyglycerol mono ester andalkyl polyglycerol diester and mixtures thereof and represented by therespective formulaeR—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂OH andR—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂—O—CO—R wherein Ris an alkyl group of 2 to 26 carbon atoms and n is an integer from about0 to 15 and mixtures thereof.
 2. The improvement of claim 1 wherein theemulsion further comprises up to 20 wt % alcohol based on the totalweight of the said emulsion wherein said alcohol is selected form thegroup consisting of methanol, ethanol, n-propanol, iso-propanol,n-butanol, sec-butyl alcohol, tertiary butyl alcohol, u-pentanol,ethylene gylcol, propylene glycol, butyleneglycol and mixtures thereof.3. The improvement of claim 1 wherein said hydrocarbon is in the boilingrange of −1° C. to 260° C.
 4. The improvement of claim 1 wherein saidwater is substantially free of salts of halides, sulfates and carbonatesof Group I and Group II elements of the long font of The Periodic Tableof Elements.
 5. The improvement of claim 1 wherein the emulsion is abicontinuous emulsion comprising a coexisting mixture of at least 60 vol% of a hydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion.
 6. The improvement of claim 1wherein said surfactant thermally decomposes at temperatures in therange of about 250° C. to about 700° C.
 7. A method to prepare abicontinuous emulsion comprising a coexisting mixture of at least 60 vol% of a hydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion the method comprising: mixing atmixing energy in the range of 0.15×10⁻⁵ to 0.15×10⁻³ kW/liter of fluidat least 40 wt % hydrocarbon, from 30 to 60 wt % of water, and from 0.01to 5 wt % of at least one surfactant selected from the group consistingof alkyl polyglycerol mono ester and alkyl polyglycerol diester andmixtures thereof, and represented by the respective formulaeR—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂OH andR—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂—O—CO—R wherein Ris an alkyl group of 2 to 26 carbon atoms and n is an integer from about0 to 15 and mixtures thereof.
 8. The method of claim 7 wherein mixing isconducted by an in-line mixer, static paddle mixer, sonicator orcombinations thereof.
 9. The method of claim 7 wherein said mixing isconducted for a time period in the range of 1 second to about 15minutes.
 10. The method of claim 7 wherein said surfactant is firstadded to said hydrocarbon to form a surfactant solution in hydrocarbonand the said water is then added to the said surfactant solution inhydrocarbon and mixed at mixing energy in the range of 0.15×10⁻⁵ to0.15×10⁻³ kW/liter of fluid.
 11. The method of claim 7 wherein saidsurfactant is first added to said water to form a surfactant solution inwater and the said hydrocarbon is then added to the said surfactantsolution in water and mixed at mixing energy in the range of 0.15×10⁻⁵to 0.15×10⁻³ kW/liter of fluid.
 12. The method of claim 7 including twosurfactants, a first surface second surfactant, and wherein the firstsurfactant is added to said water to form a first surfactant solution inwater, the second surfactant is added to said hydrocarbon to form asecond surfactant solution in hydrocarbon, the first surfactant solutionin water is added to the second surfactant solution in hydrocarbon andthe first and second surfactant solutions are mixed at mixing energy inthe range of 0.15×10⁻⁵ to 0.15×10⁻³ kW/liter of fluid.
 13. Abicontinuous emulsion comprising a coexisting mixture of at least 60 vol% of a hydrocarbon-in-water macro emulsion and from 1 to 40 vol % of awater-in-hydrocarbon macro emulsion prepared by mixing at mixing energyin the range of 0.15×10⁻⁵ to 0.15×10⁻³ kW/liter of fluid, at least 40 wt% a of hydrocarbon, from 30 to 60 wt % of water, and from 0.01 to 5 wt %of at least one surfactant selected from the group consisting of alkylpolyglycerol mono ester and alkyl polyglycerol diester and mixturesthereof; and represented by the respective formulaeR—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂OH andR—CO—O—CH₂—CHOH—CHOH—(CHOH—CHOH—CHOH)_(n)—CHOH—CHOH—CH₂—O—CO—R wherein Ris an alkyl group of 2 to 26 carbon atoms and n is an integer from about0 to 15 and mixtures thereof.
 14. The bicontinuous emulsion of claim 13further comprising up to 20 wt % alcohol based on the total weight ofthe said emulsion wherein said alcohol is selected from the groupconsisting of methanol, ethanol, n-propanol, iso-proponal, n-butanol,sec-butyl alcohol, tertiary butyl alcohol, n-pentanol, ethylene gylcol,propylene glycol, butyleneglycol and mixtures thereof.
 15. Thebicontinuous emulsion of claim 13 wherein said emulsion has a viscositythat increases with increasing temperature in the temperature range of15° C. to 80° C.
 16. The bicontinuous emulsion of claim 13 wherein saidemulsion has conductivity in the range of 3 to 40 mhos at 25° C.
 17. Thebicontinuous emulsion of claim 13 wherein said emulsion is stable tofreeze thaw cycles in the temperature range of −54° C. to 50° C.